1. Field of the Invention
The present invention relates to an image forming apparatus such as copiers, facsimiles and printers. More particularly, the present invention relates to a driving device for rotating a developer bearing member which is arranged so as to face an image bearing member to develop an electrostatic latent image on the image bearing member with a developer.
2. Discussion of the Background
Published Unexamined Japanese Patent Applications Nos. (hereinafter referred to as JP-As) 2001-083801 and 2002-182470 have disclosed image forming apparatus including a photoreceptor serving as an image bearing member and a plurality of developing devices which contain respective developers (different color toners) and develop electrostatic latent images with the different color toners to form multi-color images.
Developing devices using a one component developing method are typically used for such image forming apparatus. The one-component developing methods are as follows:    (1) a one component developer which is constituted by toner particles optionally including an external additive and which does not include a carrier is supplied to a developer bearing member (such as developing roller); and    (2) a toner layer having a predetermined thickness is formed on the surface of the developer bearing member using a developer thickness controlling member to develop an electrostatic latent image on an image bearing member with the toner layer, resulting in formation of a toner image on the image bearing member.
The one component developing method has the following advantages over two component developing methods which use a two component developer including a toner and a carrier:    (1) it is not necessary to control the toner concentration in the developer; and    (2) it is not necessary to provide an agitator configured to mix and agitate the toner and the carrier, and thereby the developing device can be simplified (i.e., the developing device can be miniaturized).
However, the developing devices using a one component developing method have the following drawbacks:    (1) toner particles sandwiched by the developing roller which rotates and the developer thickness controlling member are melted due to frictional heat, and thereby the toner particles tend to aggregate; and    (2) foreign materials invade the nip between the developing roller and the developer thickness controlling member, thereby causing a problem in that a portion of an image has low image density or the resultant image has a white streak due to uneven thickness of the toner layer and/or another problem in that the developing roller and the developer thickness controlling member are damaged by the foreign materials.
JP-As 2002-182470, 2001-356589 and 05-113714 have disclosed techniques in that the developing roller is reversely rotated to remove the foreign materials staying at the nip between the developing roller and the developer thickness controlling member from the nip to prevent occurrence of the above-mentioned problems.
FIG. 1 is a schematic view illustrating a background driving device for use in an image forming apparatus having a contact/separate mechanism similar to that described in JP-A 2001-083801. The background driving device includes a driving motor 31 which is provided on a housing of the main body of the image forming apparatus and has a motor gear 31A; and two first gears, i.e., an upper first gear 32A and a lower first gear 32A′ both of which are engaged with the motor gear 31A. An upper second gear 32B and a lower second gear 32B′ are engaged with the upper first gear 32A and the lower first gear 32A′, respectively.
The upper second gear 32B is engaged with a third gear 32C(K) which is fixedly mounted on a clutch 32D(K) and which is used for driving a black color (K) developing device. In addition, the upper second gear 32B is also engaged with a third gear 32C(Y) which is fixedly mounted on a clutch 32D(Y) and which is used for driving a yellow color (Y) developing device.
The lower second gear 32B′ is engaged with a third gear 32C(C) which is fixedly mounted on a clutch 32D(C) and which is used for driving a cyan color (C) developing device. In addition, the lower second gear 32B′ is also engaged with a third gear 32C(M) which is fixedly mounted on a clutch 32D(M) and which is used for driving a magenta color (M) developing device. These third gears 32C are arranged in a line in the vertical direction.
The clutch 32D(K) and a driving gear 34(K), which drives a black color (K) developing cartridge (not shown) and a black color (K) developing roller (not shown) to rotate, are coaxially mounted on a driving shaft 33(K). When the clutch 32D(K) is not engaged, the third gear 32C(K) idles. When the clutch 32D(K) is engaged, the third gear 32C(K) is connected with the driving shaft 33(K), and thereby the driving shaft 33(K) is rotated.
As mentioned above, the driving gear 34(K) is fixedly mounted on the driving shaft 33(K). A driven gear 35(K) which is illustrated by a dotted line in FIG. 1 is engaged with an upper portion of the driving gear 34(K). The driven gear 35(K) is fixedly mounted on a rotating shaft 38(K) of the black color developing roller (not shown in FIG. 1).
When the clutch 32D(K) is engaged, the driving force of the third gear 32C(K) is transmitted to the driving shaft 33(K). Thereby the driving gear 34(K) is rotated, and the driving force is transmitted to the driven gear 35(K), resulting in rotation of the black color developing roller. At the same time, the black color developing cartridge is moved toward an image bearing member (i.e., a photoreceptor) due to the tangential force caused by engagement between the driving gear 34(K) and the driven gear 35(K).
Similarly to the above-mentioned driving operations of the black color developing roller and the black color developing cartridge, the developing rollers and developing cartridges for use in cyan (C) color, magenta (M) color and yellow (Y) color development operations are driven.
FIG. 10(A) is a schematic view illustrating the developing operation of a background image forming apparatus having the above-mentioned background driving device. As illustrated in FIG. 10(A), the driving motor 31 is forwardly rotated at a predetermined rotational speed while the clutches K, M, Y and C are sequentially engaged for a predetermined time to perform four color development operations. After the four color development operations, the driving motor 31 is stopped, followed by reverse rotation of the driving motor 31 at a rotational speed the same as or lower than that for the forward rotation. In this case, the clutches K, M, Y and C are sequentially engaged to remove foreign materials staying at the nip between the four color developing rollers and the developer thickness controlling members.
However, when image forming operations are continuously performed several times, the foreign material removing operation (i.e., the reverse rotation operation of the developing roller) has to be performed at the end of each image forming operation, and thereby the image forming speed is decreased.
In attempting to solve the problem, JP-A 05-113714 discloses a technique in that the foreign material removing operation is performed after performing the image forming operations predetermined times. However, the problems in that the image qualities deteriorate and/or the resultant images have white streaks often occur during a time period between a foreign material removing operation and the next foreign material removing operation.
Because of these reasons, a need exists for a developing device which can form visual images at a high speed without causing image quality deterioration problems such as formation of the white streak images.